ALBERT

Notes: Optimized metal-ligand (M-L) bond lengths for 17 classical Werner-type transition-metal complexes were calculated using the local density approximation (LDA) and a gradient-corrected (GC) extension. GCs lengthen the bonds by between 0.02 and 0.09 Å relative to the LDA results. The latter range from 0.02 Å shorter than observed to 0.05 Å longer, while the GC data range from exact agreement with experiment to some 0.12 Å too long. The LDA rms deviation is 0.025 Å compared to the GC error of 0.070 Å. In contrast, data from the literature for organometallic species show that the LDA gives systematically too short M-L distances and GCs lead to a better agreement with experiment. The relative performance of LDA and GC functionals reflects the qualitatively different chemistries of organometallic and Werner-type complexes. The magnitude of the GC bond-length expansion for the latter correlates with the ionicity of the M-L interaction. © 1997 John Wiley & Sons, Inc.

Notes: The correlation-function potential-harmonic and generalized Laguerre function method (CFPHGLF) proposed recently by us is used to directly solve the Schrödinger equations of low-lying triplet states n3S (n = 2-5) for a set of heliumlike systems, including He, Li+, and Be2+. The eigenenergies converge fast and steadily with potential harmonics (PH) and generalized Laguerre functions (GLF). With 10 PH, the percentage errors in the convergent ionization energies for 23S, 33S, 43S, and 53S states of the helium atom are 0.548, 0.291, 0.247, and 0.265% relative to the Hylleraas CI variational values. Somewhat better precision is achieved for Li+ and Be2+ systems. © 1997 John Wiley & Sons, Inc.

Notes: Intermediate neglect of differential overlap (INDO) calculations were used to study the structure of C70CH2. It was found that the CH2 group is mainly added to the CI(SINGLE BOND)CII (CI represents the first kind of carbon atom and so on) or the CIII(SINGLE BOND)CIII bond in C70 and a cyclopropane feature with Cs symmetry is formed in the structure, which is in agreement with Smith et al.'s experiment. The electronic spectra of eight isomers of C70CH2 were calculated based on the optimized geometries. It was shown that the UV/vis spectra of C70CH2 and C70 resemble each other in many ways with the exception of the absorptions beyond 500 nm. The red-shift of the absorptions and NMR spectra of C70CH2 are discussed. © 1997 John Wiley & Sons, Inc.

Notes: Previous ab initio computations revealed that the conformational building unit of the right-handed helix (φ ≈ -54°, ψ ≈ -45°) is not an energy minimum on two-dimensional-type Ramachandran potential energy surfaces (E = E{φ, ψ}). Theoretical investigations were performed on several single-amino-acid diamides such as For-Gly-NH2, For-L-Ala-NH2, Ac-L-Ala-NHMe, and For-L-Val-NH2 containing amino acid residues (e.g., Ala) which can often be found in helices as shown by X-ray data analysis of globular proteins. The current ab initio [self-consistent field (SCF)] results (based on four different basis sets [3-21G, 4-21G, 4-21G*, and 6-31G*]) presented point toward an intrinsic (i.e., non-environmental-assisted) stability of the right-handed helical subconformation of a simple amino acid diamide if the residue contains a polar side chain. Such is the case for a serine derivative when its (SINGLE BOND)CH2OH side chain is favorably oriented. For the For-L-Ser-NH2 model compound two slightly different right-handed helical backbone conformations were determined. Depending on the relative orientation of the side chain, the conformational monomer of the 310 helix (a sharper helical structure with an [i, i + 3]-type H-bond network) as well as the building block of the “standard” α-helix (the regular helical structure with an [i, i + 4]-type H-bond network) were determined computationally by geometry optimization. © 1997 John Wiley & Sons, Inc.

Notes: In this work, the CLOPPA approach is extended to allow the bond-contribution analysis of the anisotropic part, ΔJ, of the indirect nuclear spin-spin coupling tensor, J. As an example this method is applied to study the role played by the X-Y bond, its antibonding orbital, and the Y lone pairs in determining the Fermi contact-spin dipolar cross contribution to Δ1J(XY) in compounds of type Me3X(SINGLE BOND)Y (X = C, Si, Sn, Pb; Y = F, Cl). That contribution is found to be the main one, except for × = Pb, in this series of compounds. Calculations are carried out using semiempirical AM1 ground-state wave functions. © 1997 John Wiley & Sons, Inc.

Notes: We present an analysis of local or semilocal density functionals for the exchange-correlation energy by decomposing them into their gradients rs (local Seitz radius), ζ (relative spin polarization), and s (reduced density gradient). We explain the numerical method pertaining to this kind of analysis and present results for a few atoms and ions. The atomic shell structure is prominent, and only the ranges 0 〈 rs 〈 10 and 0 〈 s 〈 3 are important. The low-density and large-gradient domains, where the approximations for the exchange-correlation energy are least trustworthy, have very little weight. © 1997 John Wiley & Sons, Inc.

Notes: Ab initio calculations are used to compute the change in the vibrational frequency of CN with an applied electric field aligned parallel to the bond axis. The effect of the field on the CN- vibrational frequency is of the same magnitude as for CN, but in the opposite direction. These results are compared to previous results for CO. © 1997 John Wiley & Sons, Inc.

Notes: Electron exchange between allyl molecules is studied using quantum chemical ab initio methods in order to model interchain electron transfer (ET) in polyacetylene. The reaction path, reorganization energy for ET (λ), and the electronic factor (Δ) are calculated using the UMP2, CASSCF, CASPT2, and CASSI methods. Changes in bond lengths are very small and their contribution to the internal reorganization energy almost negligible. Only changes in the bond angle leave significant contributions to the reorganization energy. The calculated potential energy surfaces can be represented as interacting parabolic-well potentials only in the case of CASSI. The UMP2, CASSCF, and CASPT2 methods give the localization of the electron even in the symmetric case and incorrect energy surfaces, with a cusp at the symmetry point. The electronic factor Δ is calculated using the two different methods, Koopmans' theorem and CASSI, with the same basis set. They are quantitatively very close for all intermolecular distances under study. However, Δ is sensitive to the basis set and cannot be calculated accurately in the present case. © 1997 John Wiley & Sons, Inc.

Notes: A theoretical investigation on the rates of electron-transfer processes Q-I + QII → Q-I + Q-II and Q-I + Q-II → QI + Q2-II was carried out by using the Marcus theory of long-range electron transfer in solution. The molecular reorganizational parameter λ, the free-energy change ΔG0 for the overall reaction, and the electronic matrix element HDA for these two processes were calculated from the INDO-optimized geometries of molecules QI, QII, and histidine. QI and QII are plastoquinones (PQ) which are hydrogen-bonded to a histidine each, and the two histidines may or may not be coordinated to a Fe2+ ion. The plastoquinone representing QI is additionally flanked by two peptide fragments. Each of the species (Pep)2QI · His and His · QII has been considered to be immersed in a dielectric continuum that represents the surrounding molecules and protein folds. INDO calculations confirm the standard reduction potential for the first process (calculated 0.127 V; observed 0.13 V) and predict a midpoint potential of 0.174 V for the second process at 300 K at pH 7 (experimental value remains uncertain but is known to be close to 0.13 V). The plastoquinone fragment carries almost all the net charge (about 95.7%) in [PQ · His]- and the net charge in [PQH · His]-. The electron is transferred effectively from the plastoquinone part of [(Pep)2QI · His]- to the plastoquinone moiety of QII · His in the first step and to the plastoquinone fragment of HisH+ · Q-II in the second step. Therefore, we made use of the formula for the rate of through-space electron transfer from QI to QII (and to Q-II). The plastoquinones are, of course, electronically coupled to histidines, and the transfer is, in reality, through the molecular bridge consisting of histidines and also Fe2+. The through-bridge effect is inherent in our calculation of ΔG0, HDA, and the reorganization parameter λ. We investigated the correlation between half-times for the transfer and (D-1op- D-1s), where Dop and Ds are, respectively, optical and static dielectric constants of the condensed phase in the vicinity of the plastoquinones. We found that with reasonable values of Dop (2.6) and Ds (8.5) the experimental rates are adequately explained in terms of transfers from the plastoquinone moiety of QI to that of QII. The t1/2 values calculated for the two processes are 247 and 472 μs in the absence of Fe2+ and 134 and 181 μs in the presence of Fe2+. These are in good agreement with the observed values which are ≈ 100 and ≈ 200 μs when Fe2+ is present in the matrix and which are known to be almost twice as large when the Fe2+ is evicted from the matrix. The present work also shows that the Marcus-Hush theory of long-range electron transfers can be successfully applied to the investigation of processes occurring in a semirigid condensed phase like the thylakoid membrane region. © 1997 John Wiley & Sons, Inc.

Notes: Knowledge of asymptotic conditions on exchange allows for a better design of exchange energy expressions in density functional theory. By working inside an exchange-only framework, the fulfillment of such conditions by some of the most widely used exchange functionals is discussed. In turn, we propose a model expression which partially meets the energetics and asymptotics of both the exchange energy density and potential. Improvement upon the local spin density approximation without the use of generalized gradient corrections is also presented. Hartree-Fock orbitals are employed to build electron densities. © 1997 John Wiley & Sons, Inc.